碳循环是一种生物地质化学循环，指碳元素在地球上的生物圈、岩石圈、水圈及大气中交换。碳的主要来源有四个，分别是大气、陆上的生物圈（包括淡水系统及无生命的有机化合物）、海洋及沉积物。

碳循环示意图。黑色数字表示碳的蕴藏量，以十亿吨的缩写。约为2004年数据计。紫色数字表示碳每年的流动量。图中的“沉积物”不包括碳酸盐及岩干酪根碳循环示意图。黑色数字表示碳的储存量，以十亿吨据计。紫色数字表示碳每年的流动量。图中的“沉积物”不包括碳酸盐及岩干酪根

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Along with the nitrogen cycle and the water cycle, the carbon cycle comprises a sequence of events that are key to making the Earth capable of sustaining life; it describes the movement of carbon as it is recycled and reused throughout the biosphere, including carbon sinks.